This invention relates to salts of heterocyclic diols useful as transesterification catalysts in melt polymerization reactions of dihydroxy aromatic compounds with diaryl carbonates. The invention further relates to a method for the preparation of polycarbonates using alkali and alkaline earth metal salts of heterocyclic diols. The method may be used to provide a product polycarbonate comprising a lower level of Fries product than is provided by other known methods employing conventional melt transesterification catalysts.
Increasingly, polycarbonate is being prepared by the melt reaction of a diaryl carbonate with a dihydroxy aromatic compound in the presence of a transesterification catalyst, such as sodium hydroxide. In this xe2x80x9cmeltxe2x80x9d process, reactants are introduced into a reactor capable of stirring a viscous polycarbonate melt at temperatures in excess of 300xc2x0C. Typically, the reaction is run at reduced pressure to facilitate the removal of by-product hydroxy aromatic compound formed as the diaryl carbonate reacts with the dihydroxy aromatic compound and growing polymer chains.
The Fries rearrangement is a ubiquitous side reaction taking place during the preparation of polycarbonate using the melt process. The resultant xe2x80x9cFries productxe2x80x9d serves as a site for branching of the polycarbonate chains thereby affecting flow and other properties of the polycarbonate. Although, a low level of Fries product may be tolerated in the product polycarbonate produced by the melt process, the presence of higher levels of Fries product may negatively impact performance characteristics of the polycarbonate, such as moldability and toughness. Currently, alkali metal hydroxides, such as sodium hydroxide, are employed as catalysts in the preparation of polycarbonate using the melt process. Alkali metal hydroxides, although effective catalysts in terms of rates of conversion of starting materials to product polycarbonate, tend to produce relatively high levels of Fries rearrangement product. Thus, melt polymerization methodology useful for the preparation of polycarbonate in which the formation of Fries product has been minimized represents a long sought goal among those wishing to practice such methodology.
It would be a significant advantage to prepare polycarbonate by a melt polymerization method which provides high rates of polymerization while minimizing the amount of Fries product formation.
The present invention provides a method for the preparation of polycarbonate, said method comprising contacting at least one diaryl carbonate with at least one dihydroxy aromatic compound in the presence of a transesterification catalyst and optionally a co-catalyst, under melt polymerization conditions to afford a polycarbonate, said transesterification catalyst comprising at least one alkali metal or alkaline earth metal salt of a heterocyclic diol.
In one aspect the method of the present invention affords a product polycarbonate having a lower level of Fries rearrangement product than polycarbonate of similar molecular weight prepared using a conventional melt transesterification catalyst.
The present invention further relates to a method of preparing polycarbonate under melt polymerization conditions by reacting at least one dihydroxy aromatic compound with at least one diaryl carbonate in the presence of at least one salt of a heterocyclic diol, said salt having structure I 
wherein A is independently at each occurrence an alkali metal ion or an alkaline earth metal ion; p is an integer having a value of 1 or 2; R1 and R2 are independently at each occurrence a halogen atom, cyano group, nitro group, C1-C20 alkyl group, C4-C20 cycloalkyl group, C4-C20 aromatic group, C1-C20 alkoxy group, C4-C20 cycloalkoxy group, C4-C20 aryloxy group, C1-C20 alkylthio group, C4-C20 cycloalkylthio group, C4-C20 arylthio group, C1-C20 alkylsulfinyl group, C4-C20 cycloalkylsulfinyl group, C4-C20 arylsulfinyl group, C1-C20 alkylsulfonyl group, C4-C20 cycloalkylsulfonyl group, C4-C20 arylsulfonyl group, C1-C20 alkoxycarbonyl group, C4-C20 cycloalkoxycarbonyl group, C4-C20 aryloxycarbonyl group, C2-C60 alkylamino group, C6-C60 cycloalkylamino group, C5-C60 arylamino group, C1-C40 alkylaminocarbonyl group, C4-C40 cycloalkylaminocarbonyl group, C4-C40 arylaminocarbonyl group, and C1-C20 acylamino group; and
X is sulfur, oxygen or the group NR3, wherein R3 is hydrogen, a C1-C40 alkyl group, a C1-C40 acyl group, C1-C40 alkoxycarbonyl group, C4-C40 cycloalkyl group, a C4-C40 aryl group.